Self-Starting electric brushless motor having permanent magnet and reluctance poles

ABSTRACT

A self-starting brushless electric motor having a first motor part (stator) ( 11 ) with poles arranged in a row, the poles constituting ferromagnetic poles ( 13 S) or permanent-magnet poles ( 14 A), a second motor part (rotor) ( 12 ) with poles arranged in a row, the poles having ferromagnetic poles ( 16 A) or permanent-magnet poles and being arranged opposite the row of poles on the first motor part ( 11 ), wherein the motor part with salient ferromagnetic poles or, if both motor parts have salient ferromagnetic poles, at least one of the motor parts has a permanent-magnet pole, and also a magnetizing winding on the first motor part. The system of poles formed by the pole rows is magnetically asymmetrical in the direction in which the motor parts ( 11, 12 ) are movable in relation to each other.

This invention relates to a self-starting brushless electric motor ofthe type which comprises reluctance poles (ferromagnetic salient poles)at least on one of the two relatively moving motor parts and one or morepermanent-magnet poles in the pole system.

Self-starting brushless electric motors can be supplied with directcurrent pulses of a single polarity or with alternating currentpolarity. When motors with moderate shaft power are suppliedelectronically, direct-current pulse supply uses the least number ofelectronic switches and thus gives the lowest system costs for motor andsupply electronics. On the other hand, for higher power, when the numberof electronic switches in the supply electronics in the motor mustanyway be increased, it may be advantageous to supply the motor withalternating current polarity so that electric power will be supplied tothe motor during both half-periods, thus achieving more uniform torquedevelopment and reducing the electrical conduction losses in thewinding.

A single-strand brushless motor has only one winding supplied from asingle external current source and provided on one of two or more partswhich are rotatable or otherwise movable in relation to each other. Sucha motor can be self-starting, i.e. develop a driving torque when atstandstill in a predetermined direction, the preferential startingdirection, only if this starting direction is inherent in the design ofthe motor. Self-starting in the preferential direction may be built intothe motor by providing asymmetry in the soft-magnetic iron core, e.g.through the use of asymmetrical salient poles and/or asymmetricalpermanent-magnet poles, or by providing auxiliary windings with noconnection to external current sources, e.g. short-circuited currentpaths as in known shaded-pole motors. Such current paths can onlyconduct current under the influence of a varying magnetic field linkedto these current paths. In order for current to flow in such currentpaths when the motor is stationary, the winding connected to an externalcurrent source must be supplied with a pulsed or alternating current.

It can be shown theoretically that motors that are not provided withauxiliary windings but are nevertheless able to exert torque in anyrotor position even when the motor winding is deenergized must alwayscontain a permanent-magnet pole.

In the following the description is limited to motors for rotarymovement which have a first part, in the following called the stator,provided with a winding, and a second part, in the following called therotor, arranged inside the stator and rotatable in relation thereto. Itwill, however, be appreciated that these two parts may exchange places,that the air gap separating the stator from the rotor need not becylindrical but may equally well be flat or conical, and that therelative movement between the parts of the motor need not be rotary butmay equally well be linear or a combination of rotary and linear, i.e.occur simultaneously about and along an axis of rotation.

The function of the motor may be described as comprising work cycleswhich are repeated a given number of times for each revolution. Atextremely low speed, e.g. when starting up from standstill, the workcycle for a motor designed to be supplied with DC pulses of one polarityconsists of one part when the winding carries current and another partwhen the winding is currentless. For a motor designed to be suppliedwith current pulses of alternating polarity the work cycle consists ofone part when the winding is supplied with current of one polarity,followed by a currentless part and thereafter a part when the winding issupplied with current of opposite polarity followed by anothercurrentless part of the work cycle.

In the currentless state the rotor must reach a starting position, i.e.a position in which the winding, if supplied with current, gives rise toa driving torque, namely a torque in the preferential direction of themotor, that is sufficiently high to overcome any frictional torque orthe like in the motor and/or in the object driven by the motor. Thetorque generated in the motor through permanent-magnetic forces mustmaintain its direction and be of sufficient strength until the rotorreaches a position in which the winding can be energized. It will beunderstood that the demand for torque development in currentless statemeans that the motor must include at least one permanent-magnet pole.

Motors operating in accordance with the principles described andexhibiting magnetic asymmetry in the pole system are known throughWO90/02437 and WO92/12567. An object of the present invention is toobtain improvements in motors of the type represented by the motors inthe aforesaid publications.

This object is achieved by means of the arrangement of magneticallyactive stator and rotor elements (poles).

Besides the opportunity of realizing constructionally alternativeembodiments, the invention also offers the opportunity of increasing theforce generated by the motor—torque in a rotating motor and linearlyacting force or “tractive force” in a linear motor—in one or morerespects:

Increasing the torque generated by permanent-magnet poles that pulls therotor of the currentless motor to the nearest starting position. Suchimprovement is advantageous in applications where high frictional torquemay appear in the driven object, for example in shaft seals.

Increasing torque appearing in a motor whose rotor is stationary in astarting position and whose winding is supplied with the highest currentavailable. Such improvement is also advantageous in the situationsmentioned in the preceding paragraph.

Increasing, at least in certain embodiments, the air-gap power of themotor for given heat losses, thereby giving a smaller and economicallymore favourable motor for a given purpose, which may be a greatadvantage when low motor weight is of importance for certain types ofapplications, e.g. in hand-held tools or other hand-held objects, but isalso an economic advantage in general, provided an unavoidable costincrease in the supply electronics does not cancel the effect.

The magnetically active elements in the motor of relevance to theinvention are as follows:

Coils on the Stator

In principle the coils form a single current circuit and may beconnected in series and/or in parallel. When the supply electronicsconsist of several units operating in parallel, these may be connectedeach to its own coil or group of coils, as if they formed a singleelectrical circuit. Instead of supplying the winding with alternatingcurrent polarity a two-part winding can be used, the two winding halvesbeing supplied with a single current polarity, but the winding halveshaving magnetically opposite directions.

Ferromagnetic Salient Poles (Reluctance Poles)

In most of the motors shown according to the invention, ferromagneticsalient poles, in the following also called reluctance poles, are to befound on the stator, alone or together with permanent-magnet poles.

There may also be reluctance poles on the rotor, but preferably notmixed with permanent-magnet poles. A mixture of these pole types on therotor can be contemplated but is normally not meaningful.

The reluctance poles on both stator and rotor may be magneticallyasymmetrical. For magnetically asymmetrical stator poles the asymmetryshould be directed in the opposite direction to the preferentialdirection of motion of the motor, whereas on the rotor the asymmetryshould be in the same direction as the preferential direction of motion.

Alternatively or in addition, the reluctance poles on both stator androtor may, however, show a certain magnetic asymmetry in the oppositedirection to that described above without this making the motorinoperable.

Permanent-magnet Poles

Motors with only reluctance poles on the rotor must always be providedwith a permanent-magnet pole on the stator. The permanent-magnet poleson the stator preferably are magnetically balanced, i.e. equal in numberand size of both polarities.

In certain cases it is an advantage if the permanent-magnet poles areasymmetrical.

Motors with permanent-magnet rotor poles designed to be supplied withcurrent pulses of a single polarity must always be provided with apermanent-magnet pole on the stator. If such permanent-magnet poles areasymmetrical in shape and have a main pole part and an auxiliary polepart, their main pole part may advantageously be displaced in thedirection opposite to that of the auxiliary pole part, e.g. from aposition it would have if the pole were symmetrical, consisting only ofa main pole part.

Motors with permanent-magnet rotor poles may lack permanent-magnet poleson the stator. Such motors are self-starting only if they are suppliedwith current pulses of alternating polarity. Such motors will then havemore uniform torque development and higher average torque for givenwinding losses than the motors supplied with current pulses of a singlepolarity.

The permanent-magnet poles, both symmetrical and asymmetrical, may haveskewed ends or edges, i.e. edges running at an angle to the direction ofthe rotor axis. In some cases such skewing of the edges of thepermanent-magnet poles may be extremely beneficial to the function ofthe motor. Such skewed edges need not be embodied in geometric shapes.It is sufficient for the edges to consist of demarcation lines(demarcation zones) relating to the imprinted magnetic polarisation (in,for example, a permanent-magnet pole), i.e. they are imprinted when thepermanent-magnet poles are magnetized.

These demarcation lines for zones with the same magnetic polarizationmay run other than linearly without the function of the motor beinggreatly affected.

The magnetic asymmetry can be achieved in several ways within the scopeof the invention and the appended claims and some of them will beexplained below.

As in the prior art motors, the magnetic asymmetry aims at building thepreferential starting direction into the motor, but the magneticasymmetry in motors according to the present invention also serves otherpurposes.

Basically, an additional purpose of the magnetic asymmetry as utilizedin the present invention is to extend what is herein termed the pull-indistance. This is the distance over which a pole, a permanent-magnetpole or a magnetized reluctance pole, on one of the motor parts iscapable of attracting a pole on the other motor part sufficiently tocause the two poles to be pulled towards one another from a first stableposition, such as the indrawn position, to the next stable position,such as the starting position, in which they are mutually alignedmagnetically and, accordingly, no magnetic pull force in the directionof relative movement exists between the poles (only a magnetic pull in adirection transverse to that direction).

During this pull-in motion the permeance between the two poles or, inother words, the magnetic flux passing between them (assuming that themagnetomotive force is constant) should increase steadily to a maximumvalue occurring when the poles are magnetically aligned. An extension ofthe pull-in distance thus calls for a lowering of the mean value of therate of flux change over the pull-in distance.

Such a lowering can be accomplished by means of magnetic asymmetry, e.g.by providing on at least one of the poles an additional pole partextending in the relative preferential starting direction so that thepole will have a main pole part and an auxiliary pole part whichdetermines the preferential starting direction.

In the starting position and the indrawn position, the auxiliary polepart extends at least to a point in the vicinity of the next pole (asseen in the relative preferential starting direction) on the other motorpart and it may even slightly overlap that pole. However, an overlappingportion of the auxiliary pole part must not carry as much flux per unitlength of overlap (measured circumferentially) as overlapping portionsof main pole parts.

Assuming that in a rotary motor chosen by way of example both theleading ends and the trailing ends of both the stator poles and therotor poles extend axially, magnetic asymmetry of a stator pole could inmost cases in principle be observed in the following way. The rotor ofthe motor is replaced with a homogenous ferromagnetic cylinder of thesame diameter as the rotor and the flux density in the air gap ismeasured along an axially extending line on the cylinder surface as thecylinder is rotated to move the line in the preferential direction ofrotation past the pole. A graph showing the measured flux density (asaveraged over the length of the line) versus the angular position of theline relative to the pole would rise, more or less steadily or in moreor less distinct steps, from a point near zero at the leading end of thepole, to a roughly constant value under the main portion of the pole andthen decline steeply at the trailing end. If the pole were magneticallysymmetric instead, the graph would be symmetrical and resemble aGaussian curve.

With suitable modifications the above-described principle is applicablealso in other cases, such as when observing magnetic asymmetry of arotor pole or a pole whose leading and trailing ends do not extendaxially. For example, where the ends of the pole are skewed so that theyextend along a helical line, the observation can be made with themeasurement of the flux density taking place along a correspondinglyskewed line.

In the case of permanent-magnet poles with uniform radial dimension anduniform radial magnetic polarization, magnetic pole asymmetry can resultfrom the pole shape. For example, the leading and trailing ends of thepole may have different lengths in the axial direction of the motor. Asimilar effect can also be achieved by magnetically imprinting poleswith a corresponding shape in a ring of permanent-magnetic material ofuniform thickness. In this case the shape of the permanent-magnet ringhas nothing to do with the magnetic pattern or “magnetic shape”.

Magnetic pole asymmetry can also be achieved by providing apermanent-magnet pole with different radial dimensions at the leadingand trailing ends, respectively, (i.e. by giving the air gap at the polea width that varies in the direction of the relative movement of themotor parts) but giving it a uniformly strong magnetization over itsentire volume.

Several methods can of course be used simultaneously in order to achievemagnetic asymmetry for the permanent-magnet poles.

There are also several ways of achieving magnetic asymmetry for salientferromagnetic poles, the reluctance poles. One method is to arrange thesurface of such a pole facing the air gap asymmetrically with regard toits extension in the axial direction of the motor, in which case theentire pole surface may be situated at the same radial distance from theaxis of rotation.

Another method is to make the projection surface of the reluctance pole(the surface facing the air gap) symmetrical, but vary its radialdistance from the axis of rotation, i.e. vary the width of the air gapalong the pole surface, stepwise or continuously, in relation to animagined (cylindrical) surface on the other motor part.

A third method is to vary the magnetic saturation flux density along thepole surface. This can be achieved by using different magnetic materialsfor different parts of the salient pole, or it can be achieved byvarying the filling factor of the laminated ferromagnetic poles, or bymeans of punched recesses, for example, below the actual pole surface(so that the actual pole surface appears to be homogenous), or byvarying the radial dimension of an auxiliary pole part such that it willhave a shape resembling the profile of the curved beak of a bird.

Of course several methods of achieving magnetic asymmetry can be usedsimultaneously. The choice of how to achieve asymmetry is usuallydependent on a balance between the manufacturing costs of the actualmotor and the cost of the supply electronics, since the choice of thetype of asymmetry may affect the size of the power electronic switchelements included in the supply electronics.

As will become apparent, in motors embodying the invention magneticasymmetry may characterize not only an individual pole of a group ofpoles which are associated with a common winding coil such that allpoles of the group are subjected to the magnetic field produced uponenergization of the coil. It may also characterize the pole group andthen not only by virtue of magnetic asymmetry of one or more individualpoles but also by virtue of an asymmetrical positioning of an individualpole within a pole group or on the rotor.

A pole of a pole group on the stator is asymmetrically positioned if arotor pole is moved through a distance longer or shorter than one-halfrotor pole pitch when it is moved between a position in which it ismagnetically aligned with that stator pole and the next adjacentposition in which any pole on the rotor is magnetically aligned with astator pole of a different pole type or, in the case of a stator havingonly permanent magnets, a pole of different polarity.

In other words, a permanent-magnet pole, for example, on the stator isasymmetrically positioned with respect to a reluctance pole in the sameor a different pole group if a rotor pole traverses a distance which islonger or shorter than one-half rotor pole pitch when the rotor movesbetween a position in which a rotor pole is magnetically aligned withthat permanent-magnet pole, i.e. is in the starting position, to thenext following or next preceding position in which a rotor pole—whichmay be any rotor pole—is in an indrawn position.

In a corresponding manner, magnetic asymmetry resulting from asymmetricpositioning of poles may also exist in the rotor. For example, in a polerow on a rotor comprising permanent-magnet poles of alternatingpolarity, the North-pole permanent-magnet poles may be displaced ineither direction from a central position between the South-polepermanent-magnet poles with all like poles substantially equally spaced.

It should be noted in the context of the present invention a pole group(pole unit) may comprise a single pole or a plurality of polesassociated with a magnetizing coil.

The invention will now be described in more detail with reference to anumber of exemplifying embodiments shown schematically in theaccompanying drawings.

FIG. 1A shows an end view of a first embodiment of a rotating motor. Thestator thereof has two identical diametrically opposed pole groups, eachconsisting of two symmetrical reluctance poles and an asymmetricalpermanent-magnet pole placed between them, provided with a magnetizingwinding. The rotor, shown in fully indrawn rotary position, has fourasymmetrical reluctance poles;

FIG. 1B is a developed view of a section of the rows of poles in themotor viewed from within the air gap and axially displaced from theirworking position, but otherwise in the position in relation to eachother that they assume in FIG. 1A;

FIG. 1C shows the motor in the same way as FIG. 1A, with thepermanent-magnetic field lines in the stator and rotor inserted;

FIG. 1D is a longitudinal sectional view of the motor of FIG. 1;

FIG. 1E is a view similar to FIG. 1B but shows the stator and rotorpoles in the same axial position and displaced to a different relativeposition and also includes a graph representing the magnetic attractionforce acting between a permanent-magnet pole on the stator and areluctance pole on the rotor.

FIG. 1F is similar to FIG. 1E but shows the starting position. It doesnot show the rotor in cross-section, but instead shows the rotor indash-dot outline.

FIGS. 2A, 2B and 2C show a second embodiment in corresponding manner toFIGS. 1A, 1B and 1C;

FIGS. 3A, 3B to 12A, 12B show further embodiments in the correspondingmanner to FIGS. 1A and 1B.

FIGS. 13A to 13D are developed views resembling FIG. 1B of polecombinations which differ from each other in respect of the shape andplacement of a permanent-magnet pole on the stator.

FIGS. 14A, 14B are fragmentary views showing a modified form of thestator reluctance poles of the motor shown in FIGS. 1A, 1B.

Throughout the drawings, the polarity of the radially magnetizedpermanent-magnet poles is indicated by an arrow-head pointing towardsthe North-pole side of the magnet.

Moreover, in all embodiments shown in the drawings, the asymmetry of thestator and/or the rotor poles is directed such that the preferentialstarting direction of the rotor is counterclockwise.

The motor shown in FIGS. 1A-1C is a rotating motor, as are the othermotors also, with a first motor part in the form of a laminatedferromagnetic stator 11 and a second motor part in the form of alaminated ferromagnetic rotor 12 journalled for rotary movement in thestator by suitable bearings as shown in FIG. 1D. The axis of rotation ofthe rotor is indicated by a small circle, designated 12A and thepreferential direction of rotation is indicated by an arrow(counterclockwise in all illustrated embodiments).

The stator 11 has two diametrically opposite pole groups. Each polegroup comprises two salient ferromagnetic poles 13S, also calledreluctance poles, spaced from each other circumferentially with apermanent-magnet pole 14A arranged between them. The surfaces of thesepoles 14A facing the rotor are located on a cylindrical surface that isconcentric with the axis of rotation 12A of the rotor.

For each pole group the stator 11 is also provided with a coil 15 woundaround the pole group and forming part of a common magnetizing winding.

On the outside of the rotor 12, distributed uniformly around itsperiphery, are four salient ferromagnetic poles 16A, also calledreluctance poles. The surfaces of these poles facing the stator arelocated on a cylinder that is concentric with the axis of rotation 12A,a short distance from the cylinder containing the pole surfaces of thestator, so that the pole surfaces of the stator and those of the rotorform an air gap 17 between them. The pole pitch of the rotor 12corresponds to the spacing of the reluctance poles 13S within each polegroup on the stator 11.

In the embodiment shown in FIGS. 1A to 1D, all the poles 13S, 14A on thestator 11 and the poles 16A on the rotor 12 are located in the sameplane perpendicular to the axis of rotation, so that during rotation allpoles on the rotor pass over and interact with all poles on the stator.The motor may of course have several axially separated sets of polegroups arranged in this manner. Furthermore, instead of being located inclosed paths or rows running peripherally around the rotor, the poles oneach motor part may be arranged, for example, on helical paths.

The pole surfaces on the reluctance poles 13S of the stator aremagnetically symmetrical in the sense intended in this application. Thesignificance of this is that if the rotor 12 were to be replaced by ahomogenous ferromagnetic cylinder, the envelope surface of whichcoincides with the cylindrical surface on which the rotor poles 16A areotherwise located, a magnetic field would flow through the air gap 17below and around the pole surfaces of the stator poles when the currentwas supplied to the winding 15, the magnetic field having suchdistribution that a diagram of the mean value of the magnetic fluxdensity along a generatrix on said ferromagnetic cylinder surface, drawnas a function of the angular position of this cylinder in relation tothe stator, would show symmetry of the same type as, for example, aGaussian curve, i.e. mirror symmetry about a line perpendicular to theabscissa. Said symmetry means that the shape of the diagram isindependent of which direction of rotation of the cylinder is defined aspositive.

On the other hand, the permanent-magnet poles 14A are magneticallyasymmetrical since they have a protrusion 14′ on the side facing againstthe direction of rotation of the rotor, said protrusion being caused bythe poles 14A on this side having narrower breadth, i.e. dimensionparallel to the axis of rotation 12A, than over the main part of thepoles. The part of the permanent-magnet poles 14A with full breadth maybe said to constitute a main pole part, while the narrower protrusionmay be said to constitute an auxiliary pole part, designated in thefigures by 14′.

The reluctance poles 16A on the rotor 12 are also asymmetrical incorresponding manner since they are provided on their leading orcounterclockwise side, the side directed in the direction of rotationwith a protrusion 16′ (auxiliary pole part) having a breadth less thanthat of the main part (main pole part) of the poles.

As is evident from the above, the asymmetry in the poles can be achievedin ways other than those just described. One example of an alternativemethod is indicated in dash-dot lines in FIGS. 1A and 1B. In thisalternative each pole has the same breadth across its entire axial andcircumferential dimension, but at one side the pole surface is offsetradially inwards so that the air gap 17 is larger at this side than overthe main part of the pole.

FIG. 1B shows a developed view of one of the pole groups of the statorand the rotor in FIG. 1A as viewed from within the air gap 17 and withthe rotor poles displaced axially in relation to the stator pole group.The parallel dash-dot lines R and S indicate the direction of therelative movement between rotor and stator. The dash-dot lines R and Salso are lines (alternatively described as paths, circles, or rows)along which the poles are deployed. The dash-dot line L perpendicularthereto represents the centre line between the stator poles. Theposition of the stator and rotor poles in relation to each othercorresponds to the relative position shown in FIG. 1A and is the stableposition the rotor assumes in relation to the stator when current issupplied to the winding 15 so that the reluctance poles 13S tend to keepthe rotor poles 16A in an attracted or indrawn position with the mainpole parts opposite to the reluctance poles.

When current is no longer supplied to the winding in this rotorposition, then only the permanent-magnetic flux from thepermanent-magnet poles 14A acts on the rotor to pull it further in thedirection of the starting position. FIG. 1C shows the flux pattern ofthe permanent-magnet poles in this position.

As is shown in FIGS. 1A-1D, in the indrawn position the auxiliary poleparts 16′ of two of the rotor poles 16A, the upper right and the lowerleft rotor poles, extend up to the auxiliary pole parts 14′ of thepermanent-magnet poles 14 and preferably even overlap the auxiliary poleparts slightly. This relative position of the permanent-magnet poles andthe auxiliary pole parts of the rotor poles is a position in which themagnetic attraction force the permanent-magnet poles 14 exert on theserotor poles, and hence the counterclockwise torque applied to the rotor12 by the permanent-magnet poles 14A, is at or near its maximum, thewinding coils being currentless.

At the same time, the spacing of the other two rotor poles 16, the upperleft and the lower right rotor poles, from the permanent-magnet poles14A is substantial so that the permanent magnet-poles only apply aninsignificant clockwise torque to the rotor.

Accordingly, the net counterclockwise torque applied to the rotor by thepermanent-magnet poles 14A is capable of forcefully jerking the rotor 12counterclockwise from the indrawn position and turn it through an anglecorresponding to one-half of the rotor pole pitch to bring the rotorpoles 16A to the starting position (pull-in movement).

Throughout the pull-in movement of the rotor from the indrawn positionto the starting position the magnetic flux between each permanent-magnetpole 14A and the rotor pole which it overlaps and with which itinteracts increases steadily with increasing pole overlap so that acounterclockwise torque is exerted on the rotor 12 until the startingposition has been reached.

In the starting position each of the two first-mentioned rotor poles 16Ais magnetically aligned with respectively the upper and the lowerpermanent-magnet pole 14A, a portion of the auxiliary pole part 16′ onthe leading end of the rotor pole 16A extending in the counterclockwisedirection beyond the permanent-magnet pole and the trailing end of therotor pole being positioned opposite to the auxiliary pole part 14′ ofthe permanent-magnet pole. This is shown in FIG. 1F.

FIG. 1E includes a graph representative of an exemplary embodiment ofthe motor shown in FIGS. 1A to 1D which shows the pull-in force F actingbetween the permanent-magnet poles 14 and the rotor reluctance poles 16as a function of the overlap d of the leading end 16″ of the rotorreluctance pole 16 and the corresponding edge 14″ of thepermanent-magnet pole 14A, during the pull-in movement from the indrawnposition to the starting position. In the right-hand portion of FIG. 1Ethe indrawn position of the rotor reluctance pole 16A is shown as across-section. The view is looking outward through a cylindricalsurface, centered on the rotation axis 12A, slightly smaller in diameterthan the rotor. FIG. 1E shows the features of FIG. 1B, but overlapped.

The graph shows the pull-in force F acting on the rotor reluctance pole16 for different amounts of overlap (positive d) and separation(negative d) between the auxiliary pole part 14′ of the permanent-magnetpole 14A and the auxiliary pole part 16′ of the rotor reluctance poles16A in the indrawn position.

From FIG. 1E it is apparent that if the overlap in the indrawn positionis −1 mm, that is, if the leading end 16″ of the reluctance pole 16 isspaced 1 mm in the negative or clockwise direction from thepermanent-magnet pole, the pull-in force is quite small. If the leadingend 16″ is opposite the end of the auxiliary pole part 14′ (zerooverlap), the pull-in force is substantially greater, and for a positiveoverlap of about 1 mm the pull-in force on the auxiliary pole part 16′is at or near its maximum where it is three to four times the pull-inforce for a negative overlap of about 1 mm. The asymmetry of the statorpermanent-magnet pole 14 in conjunction with the asymmetry of the rotorreluctance pole 16 thus produces a dramatic increase of the initialvalue of the pull-in force in comparison with the case where only therotor reluctance pole is asymmetrical as in the motor disclosed inWO92/12567. This increase of the pull-in force broadens the field ofapplication of the motor according to the invention.

From FIG. 1E it is also apparent that as the overlap then graduallyincreases during the pull-in movement, the pull-in force remainsapproximately constant during the first portion of the pull-in movement,namely until the main pole parts begin to overlap. During the continuedpull-in movement the pull-in force will first increase and thengradually drop to zero as the reluctance pole 16A reaches the startingposition.

Moreover, FIG. 1E shows that during the counterclockwise pull-inmovement of a rotor reluctance pole 16A from the indrawn position to thestarting position, the permanent-magnet pole 14A will exert asubstantial attraction force on the rotor reluctance pole throughout acircumferential distance which is greater than the circumferentialdimension of the permanent-magnet pole 14A: from a position in which theleading end 16″ is opposite to or only slightly spaced in the clockwisedirection from the end 14″ of the auxiliary pole part 14′ of thepermanent-magnet poles 14A up to a point in which the leading end 16″ iswell past the permanent-magnet pole.

When the winding coils 15 are again energized with the rotor poles inthe starting position, the auxiliary pole part 16′ of all four rotorpoles 16A will therefore be near a stator reluctance pole 13S ahead ofit as seen in the direction of rotation of the rotor. On the other hand,the spacing of the trailing end of each rotor pole from the statorreluctance pole behind it is substantial. The magnetic attraction in thecounterclockwise direction between the stator reluctance pole 13S andthe leading end 16″ of the rotor reluctance pole 16A behind it will thusbe heavily predominant over the magnetic attraction in the clockwisedirection exerted on the trailing end of the same rotor pole by the nextstator reluctance pole (i.e. the reluctance pole behind the rotor pole).

Accordingly, the net torque applied to the rotor by the statorreluctance poles 13S will act in the counterclockwise direction and willbe high so that the motor will be capable of starting against aconsiderable load. Again, the magnetic flux produced as a consequence ofthe energization of the winding coils opposes the magnetic flux producedby the permanent-magnet poles 14A so that the permanent-magnet poles donot substantially counteract the movement from the starting position.

The embodiments illustrated in the other figures are described onlyinsofar as they differ from the embodiment shown in FIGS. 1A-1D. Thesame designations are used throughout for all embodiments, with thesuffix letter A or S indicating asymmetrical or symmetrical. Unlessotherwise stated, “symmetry” and “asymmetry” with regard to the polesrelates to their magnetic symmetry or asymmetry rather than theirgeometrical symmetry or asymmetry (which may or may not correspond tothe magnetic symmetry or asymmetry).

The motor in FIGS. 2A-2C differs from the motor in FIGS. 1A-1C only withregard to the stator poles. More specifically, one stator reluctancepole 13A in each stator pole group is magnetically asymmetrical with anauxiliary pole 13′ of the same type as the auxiliary pole 16′ on therotor poles 16A, whereas the other reluctance pole 13S and thepermanent-magnet poles 14S are magnetically symmetrical. The auxiliarypole parts 13′ amplify the above-described effect of the auxiliary rotorpole parts 16′ when the winding coils are energized with the rotor polesin the starting position.

The motor in FIGS. 3A, 3B also differs from the motor in FIGS. 1A-1Conly with regard to the stator poles. In this case the stator reluctancepoles 13A and 13S in each stator pole group are the same as in FIGS. 2A,2B, i.e. one is asymmetrical and the other is symmetrical. However, thepermanent-magnet poles 14A are asymmetrical with an auxiliary pole 14′of the same type as the rotor auxiliary pole 16′, with the asymmetrydirected in the same direction as the asymmetry of the stator reluctancepole 13A. Consequently, asymmetry exists in all three pole types in thismotor.

FIGS. 2A, 2B and 3A, 3B, as well as some of the following figures, showthat all poles of a certain type, whether reluctance poles orpermanent-magnet poles, in a pole group need not necessarily be of thesame kind in respect of symmetry or asymmetry. This is true for allembodiments.

The motor in FIGS. 4A, 4B has two symmetrical stator reluctance poles13S and one asymmetrical stator permanent-magnet pole 14A in each polegroup and, as far as the stator 11 is concerned, therefore agrees withthe motor in FIGS. 1A-1C. In this case, however, the rotor 12C isdesigned differently from the rotor 12 in the previous embodiments,partly since it has only permanent-magnet poles 18AN and 18AS,asymmetrical ones with auxiliary poles 18′ facing the same way, andpartly since these permanent-magnet poles are arranged without spacesaround the periphery of the rotor body, adjacent poles having oppositepolarities, N and S, respectively.

In the motor in FIGS. 5A, 5B each stator pole group has only tworeluctance poles, i.e. symmetrical reluctance poles 13S, and the stator11 thus lacks permanent-magnet poles. The rotor 12C is similar to therotor in FIGS. 4A, 4B except that the permanent-magnet poles 18AN, 18ASare shaped slightly differently.

In the motor in FIGS. 6A, 6B, as in the motor in FIGS. 2A-2B, the statorpole groups have one asymmetrical and one symmetrical reluctance pole13A and 13S, respectively, combined with a symmetrical permanent-magnetpole 14S, while the rotor 12C is the same as the rotor in FIGS. 4A, 4B.

The motor in FIGS. 7A, 7B has stator pole groups of the same type as themotor in FIGS. 3A, 3B, i.e. with one asymmetrical and one symmetricalreluctance pole 13A and 13S, respectively, and an asymmetricalpermanent-magnet pole 14A, and the rotor 12C is of the same design as inFIGS. 4A, 4B and 6A, 6B.

The stator pole groups of the motor in FIGS. 8A, 8B have onlyasymmetrical reluctance poles 13A and thus no permanent-magnet poles,and the rotor is very similar to that in FIGS. 4A, 4B and 6A, 6B.

In the motor in FIGS. 9A, 9B, stator pole groups are used which have apole combination corresponding to that in the motor in FIGS. 7A, 7B—anasymmetrical reluctance pole 13A, a symmetrical reluctance pole 13S andan asymmetrical permanent-magnet pole 14A—together with symmetricalpermanent-magnet poles 18SN, 18SS of alternating polarity on the rotor.

Like the motor in FIGS. 8A, 8B, the motor in FIGS. 10A, 10B has statorpole groups with only reluctance poles, i.e. asymmetrical reluctancepoles 13A with a relatively long auxiliary pole 13′. As in the motor inFIGS. 9A, 9B, the rotor 12C has only magnetically symmetricalpermanent-magnet poles 18SN, 18SS, in this case however with partiallyskewed edges.

FIGS. 11A, 11B show a motor in which the rotor 12D resembles the rotor12C in FIGS. 1A to 1D except that it is provided with three asymmetricalreluctance poles 16A. In this case the stator 11D is circular and onlyprovided with permanent-magnet poles, namely two diametrically oppositegroups of asymmetrical permanent-magnet poles 14AN, 14AS, each groupcomprising two spaced poles of alternating polarity N and S, the polepitch being one-half of the rotor pole pitch. Moreover, in this motorthe winding coils 15D are supplied with current pulses of alternatingpolarity. The winding coils 15D differ from the winding coils of thepreceding embodiments in that they are sectionally toroid-wound aroundthe stator 11D.

Other combinations of poles on stator and rotor are also possible withinthe scope of the invention. Besides the pole arrangements illustratedand described above, the following list includes examples of polearrangements falling within the scope of the invention.

I. Symmetrical Reluctance Poles on the Stator

A. Asymmetrical permanent-magnet poles on the stator (in symmetrical orasymmetrical placement)

1. Reluctance poles on the rotor

a. Asymmetrical reluctance poles FIG. 1

b. Symmetrical reluctance poles

2. Permanent-magnet poles on the rotor

a. Asymmetrical permanent-magnet poles (arranged in symmetrical orasymmetrical pole row) FIG. 4

b. Symmetrical permanent-magnet poles (arranged in symmetrical orasymmetrical pole row)

B. Symmetrical permanent-magnet poles (in asymmetrical placement on thestator)

1. Reluctance poles on the rotor

a. Asymmetrical reluctance poles

b. Symmetrical reluctance poles FIG. 12

C. Lacks permanent-magnet poles on the stator

1. Permanent-magnet poles on the rotor

a. Asymmetrical permanent-magnet poles FIG. 5

b. Symmetrical permanent-magnet poles arranged in asymmetrical pole row

II. Asymmetrical Reluctance Poles on the Stator

A. Symmetrical permanent-magnet poles on the stator

1. Reluctance poles on the rotor

a. Asymmetrical reluctance poles FIG. 2

b. Symmetrical reluctance poles

2. Permanent-magnet poles on the rotor

a. Asymmetrical permanent-magnet poles FIG. 6

B. Asymmetrical permanent-magnet poles on the stator

1. Reluctance poles on the rotor

a. Asymmetrical reluctance poles FIG. 3

b. Symmetrical reluctance poles

2. Permanent-magnet poles on the rotor

a. Asymmetrical permanent-magnet poles FIG. 7

b. Symmetrical permanent-magnet poles FIG. 9

C. Lacks permanent-magnet poles on the stator

1. Permanent-magnet poles on the rotor

a. Asymmetrical permanent-magnet poles (arranged in symmetrical orasymmetrical pole row) FIG. 8

b. Symmetrical permanent-magnet poles (arranged in symmetrical orasymmetrical pole row) Fig. 10

III. Symmetrical Permanent-magnet Poles Only on the Stator

A. Stator poles arranged in a symmetrical pole row

1. Reluctance poles on the rotor

a. Asymmetrical reluctance poles

B. Stator poles arranged in asymmetrical pole row

1. Reluctance poles on the rotor

a. Asymmetrical reluctance poles

b. Symmetrical reluctance poles

IV. Asymmetrical Permanent-magnet Poles on the Stator

A. Stator poles arranged in symmetrical Pole row

1. Reluctance poles on the rotor

a. Asymmetrical reluctance poles FIG. 11

b. Symmetrical reluctance poles

B. Stator poles arranged in asymmetrical pole row

1. Reluctance poles on the rotor

a. Asymmetrical reluctance poles

b. Symmetrical reluctance poles

FIGS. 12A and 12B show a motor which is also within the scope of theinvention. All individual poles, both those on the stator and those onthe rotor, are magnetically symmetrical. In this motor the object of theinvention is instead achieved by magnetic asymmetry within the polegroups on the stator, namely by asymmetrical positioning of asymmetrical permanent-magnet pole between a pair of symmetricalreluctance poles of the pole groups. As the rotor is also provided withsymmetrical reluctance poles, the motor shown in FIGS. 12A, 12B may beregarded as belonging to category I.B.1.b. in the above categorization.

More particularly, the motor shown in FIGS. 12A, 12B comprises a stator11 having reluctance poles 13S similar to those shown in FIGS. 1A to 1D.The rotor 12 also resembles the rotor in FIGS. 1A to 1D, except that itspoles 16 are provided with auxiliary pole parts 16′ both at the leadingend and at the trailing end and these auxiliary pole parts are of lessercircumferential dimension.

Each pole group comprises a rectangular magnetically symmetricalpermanent-magnet pole 14 which is placed in an asymmetric or off-centreposition adjacent to one of the two reluctance poles 13S. The twopermanent-magnet poles 14 are connected to a common actuating mechanism20 including a lever 21. In operation of the motor the permanent-magnetpoles 14 are stationary in the selected off-centre position, but byshifting the lever 21 downwards from the position shown in full lines inFIG. 12A, the permanent-magnet poles 14 can be moved circumferentiallyfrom the illustrated off-centre position to a corresponding off-centreposition (indicated in dash-dot lines in FIGS. 12A, 12B) adjacent to theother stator reluctance poles 13S to reverse the preferential directionof rotation of the rotor.

The rotor 12 is shown with its poles 16 in the indrawn or attractedposition. In this position the auxiliary pole parts 16′ at the trailingend (assuming counterclockwise rotation of the rotor) of two of therotor poles, the upper left pole and the lower right pole, is closelyadjacent to one end of each permanent-magnet pole 14 and preferablythere is a slight overlap between each permanent-magnet pole and theadjacent one of these rotor poles. The spacing of the opposite side ofeach permanent-magnet pole 14 and the other adjacent rotor pole issubstantial.

Accordingly, in the illustrated indrawn rotor position, the magneticattraction between each permanent-magnet pole 14 and the rotor pole 16ahead of it, as seen in the direction of rotation of the rotor, will beheavily predominant over the magnetic attraction between thepermanent-magnet pole and the rotor pole behind it. When the current inthe winding coils 15 is switched off with the rotor in the illustratedposition, the permanent-magnet poles 14 therefore will pull the rotorclockwise to the starting position.

In the starting position the auxiliary pole parts 16′ on the leading endof the rotor poles 16 will be closely adjacent to, and preferablyslightly overlap, the two stator reluctance poles 13S ahead of them.When the winding coils 15 are then again energized, these reluctancepoles can therefore forcefully jerk the rotor in the counterclockwisedirection away from the starting position as described above withreference to FIGS. 1A to 1E.

FIGS. 13A to 13D are views corresponding to FIGS. 1B, 2B, 3B etc. andserve to further elucidate the concept of magnetically symmetrical andasymmetrical positioning of a permanent-magnet pole between a pair ofreluctance poles on the stator. These four figures show four differentshapes of the permanent-magnet pole together with a stator and rotorreluctance pole combination which is the same in all figures and similarto that in FIGS. 1A and 1B. All four figures show the rotor reluctancepoles 16A in the starting position, that is magnetically aligned withthe permanent-magnet pole 14S (FIG. 13A) or 14A (FIGS. 13B to 13D), thewinding associated with the stator pole group being currentless.

In the starting position the force exerted on the rotor reluctance pole16 by the permanent-magnet pole 14S, 14A is zero in the circumferentialdirection but in response to any deviation of the rotor pole from thealigned position the attraction force between the permanent-magnet poleand the rotor pole will develop a circumferential component tending toreturn the rotor pole to the starting position.

In FIG. 13A, which is included for comparison and shows a symmetricalpole configuration corresponding to that shown in WO92/12567, thepermanent-magnet pole 13S completely overlaps the main pole part 16A ofthe rotor reluctance pole 16. FIGS. 13B-13D show different stator poleconfigurations according to the invention which are asymmetrical byvirtue 35 of asymmetrical shape of the permanent-magnet pole (FIGS.13B-D) and/or by virtue of asymmetrical positioning thereof (FIG. 13D).

In FIG. 13D the stator pole group consisting of the poles 13S and 14A isasymmetrical both by virtue of pole asymmetry of the permanent-magnetpole 14 and by virtue of a slightly asymmetric positioning of that pole(towards the left stator reluctance pole 13S), resulting in a slightlyasymmetrical aligned position of the rotor reluctance pole 16A betweenthe two stator reluctance poles 13S. Consequently, the distance therotor pole 16A traverses when it moves from the position in which it ismagnetically aligned with the right stator reluctance pole to theposition in which it is magnetically aligned with the permanent-magnetpole 14A is slightly longer than the distance it traverses from thelast-mentioned position to the position in which it is magneticallyaligned with the left reluctance pole 13S.

Although magnetic asymmetry on the rotor resulting from asymmetricalpositioning of poles is not shown in the drawings, such asymmetry, aloneor in combination with asymmetry of individual poles, such asymmetry inthe stator-rotor pole system is also possible. For example, in rotors ofthe kind shown in FIGS. 4A, 4B to FIGS. 10A, 10B, in whichpermanent-magnet poles of one polarity alternate with permanent-magnetpoles of the other polarity, the permanent-magnet poles of one polaritymay be collectively displaced in either circumferential direction fromthe central position between adjacent permanent-magnet poles of theother polarity, the poles within each set of like-polarity poles stillbeing substantially evenly spaced-apart.

In all embodiments illustrated in the drawings, the stator and the rotorare laminated from thin electrical-steel plates as is indicated in FIG.1D (where the thickness of the plates is heavily exaggerated forclearness of illustration).

In the portions of the plates which form the stator reluctance poles 13Sor 13A, every second stator plate 11A is slightly reduced such that thecurved plate edges 11B facing the air gap 17 are offset radiallyoutwardly relative to the neighbouring plates, see FIG. 1A and the lowerportion of FIG. 1D. In other words, only every second plate 11C extendsup to the air gap 17 while the intervening plates 11A end short of theair gap 17. A similar reluctance pole design is or may be provided inthe stator and or the rotor of all those motors in which both the statorand the rotor are provided with reluctance poles.

This thinning out of the plate stack at the pole face of the reluctancepoles serves to ensure that the change of the flux in the air gapbetween the stator and rotor reluctance poles that takes place as therotor reluctance poles move past the stator reluctance poles isproportional to the change of the pole overlap area. In other words,they serve to ensure that the flux density in the pole overlap area issubstantially constant as long as the flux change is not limited bymagnetic saturation in a different region of the magnetic circuit sothat the torque developed by the interaction of the poles will be asuniform as possible.

Magnetically, the effect of the reduction or shortening of thereluctance pole portion of every second plate is a 50% lowering of theaveraged value of the saturation flux density across the pole faceserving the purpose of reducing the magnetic induction swing (theinterval in which the flux density varies over an operating cycle of themotor) in the bulk of the laminated stack, where the predominant part ofthe iron losses arise.

FIGS. 14A and 14B show a modified technique for thinning out thereluctance poles at the pole faces. This modified technique, which issuitable for motors running at elevated operating frequencies, is notlimited to the single-phase motors described above but is generallyapplicable to all motors having reluctance poles both on the stator andthe rotor. For example, motors of the kinds disclosed in WO90/02437 andWO92/12567 can have reluctance poles of the stator and/or the rotordesigned according to the modified technique.

Increased motor speeds require increased operating frequencies of thecurrent supply for the motor. However, increased operating frequenciesare accompanied by increased iron losses. One technique for avoiding theincrease of the iron losses consists in using thinner plates for thelaminations, but if the plate thickness is reduced it may be difficultor impossible to use automatic production equipment. Another techniqueconsists in reducing or shortening two out of every three plates, butthis technique is in most cases unsatisfactory.

It is also an object of the present invention to provide a reluctancepole design which can be adapted for motors operating at elevatedfrequencies without it being necessary to resort to any of theabove-described techniques.

In accordance with this aspect of the invention, the desired reductionof the induction swing at increased operating frequencies is achieved ina reluctance pole of the type shown in FIGS. 1A-1D by providing recessesin those plates which extend up to the air gap, which recesses constrictthe cross-sectional area of the plate presented to the magnetic flux inthe pole and thereby contribute to a lowering of the flux density forwhich the pole becomes magnetically saturated at the pole face.

The recesses should be distributed substantially uniformly over thecross-section of the plate. They may take the form of holes, i.e.openings which are not open to the air gap, or they may take the form ofopenings which communicate with the air gap, preferably via narrowpassages. Wide passages are undesirable because they give rise to eddycurrents in the faces of the reluctance poles of the other motor part.

In FIGS. 14A and 14B the modification is exemplified for the reluctancepoles of the stator of the motor shown in FIGS. 1A-1D, namely the statorreluctance pole 13S to the right in the upper stator pole group. FIG.14A shows the shortened reluctance pole portion of one plate 11A whileFIG. 14B shows the full-length reluctance pole portion of theneighbouring plate 11C. In the region near the air gap 17 this portionis provided with three recesses 11D in the form of elongate openingswhich have a closed contour and thus are not connected with the curvededge 11E facing the air gap 17. The three recesses are uniformlydistributed along the length of the curved edge.

When designing the recessed portion of the plates, the followingempirical equation

ΔB ₂ =ΔB ₁(f ₁ /f ₂)^({fraction (1/1.2)})

is helpful. In this equation, ΔB and f represent respectively theinduction swing and the operating frequency, the indices 1 and 2denoting two different operating conditions. As is immediately apparentfrom the equation, an increased operating frequency with unchanged ironlosses calls for a reduction of the induction swing which is less thandirectly proportional to the increase of the operating frequency. Forexample, a doubling of the operating frequency requires a reduction ofthe induction swing to 56% of its previous value for the iron losses toremain unchanged. At elevated operating frequencies it becomes possibleto adjust the iron and copper losses so that they become approximatelyequal which is optimal for torque generation. Consequently, the fluxdensity may be chosen higher than the flux density corresponding tounchanged iron losses.

Although the above-described recessing of the iron plates lowers thesaturation flux density at the reluctance pole faces, a substantialincrease in air-gap power for the same motor size may be achievedbecause the motor speed can be increased more than the motor torque mustbe decreased.

Naturally, the recessing of the reluctance pole portions of the platesin accordance with the principle described above can be applied tomotors in which the reluctance pole portions of all plates extend up tothe air gap as shown in respect of the plates 11C in FIGS. 1D and 14B.If desired, the recessing may be different for neighbouring plates.

Alternative Embodiments

The following alternative motors are assembled from pole groups withwindings and may be shaped for rotary or linear movement.

The rotating motors may have

1. An air-gap surface that is cylindrical, conical, disc-shaped, etc.,in principle any shape of surface that a generatrix rotating about astationary axis can describe.

2. External rotor.

3. The difference between the pole number in the stator and rotor,respectively may be arbitrary, e.g. in the case of segmented stator(s).

4. In a motor with cylindrical air-gap surface and internal rotor,several pole groups may be arranged so that they are connected togetherby electrical steel laminations in the same plane (as in the embodimentsillustrated in the drawings). A motor may consist of several such “motordiscs” along a common axis of rotation.

These discs may alternatively be designed without individually closedflux paths and are instead connected by axially directed flux paths.Examples of such an arrangement can be found in W090/02437. The “motordiscs” can be magnetized by common coils for two “motor discs” forexample.

5. For motors with axial flux connection between “motor discs”, thewinding may consist of a cylinder coil surrounding the axis of rotation(an example of such an arrangement is shown in WO90/02437). In thiscase, for example, the rotating part may contain the pole types whichwould otherwise have been stationary, and vice versa.

6. Motors in which the stator does not have both reluctance poles aswell as permanent-magnet poles, and thus only one pole type, can bemodified within the scope of the invention by having the stator polesexchange places with the rotor poles. Examples of such cases are themotors in FIGS. 5A, 5B, 8A, 8B and 10A, 10B. In these motors, thus, thereluctance poles on the stator can be replaced with permanent-magnetpoles corresponding to those on the rotor, and the permanent-magnetpoles on the rotor can be replaced with reluctance poles correspondingto those on the stator. FIGS. 11A, 11B show such a modification of themotor in FIGS. 8A, 8B.

7. The shape and/or distribution of the poles in a motor, e.g. thereluctance poles, may be chosen so that noise and vibration generated byvarying magnetic forces between stator and rotor are reduced as far aspossible. Examples of known measures of this type are skewed pole edgesor a slightly uneven distribution of the poles along the periphery ofthe rotor or a certain difference between the pole pitch in a pole groupon the stator and the pole pitch on the rotor. Various measures can alsobe combined.

8. In motors with two, or some other even number of reluctance poles ineach group on the stator and no permanent-magnet poles on the stator,the soft-magnetic stator yoke part which in the embodiments shown runsin the middle of the pole group, can be eliminated without affecting themagnetic function of the motor. The mechanical function of said statoryoke parts as spacers may be replaced with non-magnetic spacer means.

9. It will be understood that the coils shown in FIGS. 1 to 12 formagnetizing the pole groups can also be arranged differently, e.g. ascoils of transformer type surrounding the yokes between the pole groups,or as shown in FIG. 11A. The stator yoke may also be divided therebyenabling pre-wound coils to be used. It may also be economicallyadvantageous in small motors, for example, to replace two yokes thatconnect two pole groups together, with a single yoke with doubledcross-sectional are and have a single coil surrounding the yoke. Sucharrangements are known in small shaded-pole motors and DC motors.

10. To enable the use of only a single electronic switching element inmotors supplied with current pulses of a single polarity, field energycan be returned to the DC source by means of feedback winding wound inparallel with the operating winding, as described in WO90/02437.

What is claimed is:
 1. A self-starting brushless electric motor,comprising a first motor part (11) having a plurality of pole groupsarranged in spaced-apart relation in a first pole line, a second motorpart (16) having a plurality of poles arranged in spaced-apart relationin a second pole line, bearing means supporting the first motor part andthe second motor part for relative movement with the first pole lineconfronting the second pole line across an air gap, the first and secondpole line comprising a pole system comprising reluctance poles andpermanent-magnet poles which are polarized transversely to the air gap,and a winding system (15) on the first motor part comprising a windingcoil arranged in association with each pole group to produce a magneticfield linking the poles of the first pole line and the second pole linethrough the pole group upon energization of the coil, at least one ofthe first pole line and the second pole line including a magneticasymmetry providing a preferential relative direction of movement of themotor parts upon energization of the winding system, at least one ofsaid pole groups comprising an asymmetrical reluctance pole, and whereinsaid at least one pole group of the first pole line includes onesymmetrical reluctance pole, one asymmetrical reluctance pole and onesymmetrical permanent-magnet pole positioned between the reluctancepoles, and in which the second pole line comprises a plurality ofsubstantially evenly spaced identical poles.
 2. A motor as claimed inclaim 1, in which the second pole line only comprises asymmetricalreluctance poles.
 3. A motor as claimed in claim 1, in which the secondpole line only comprises asymmetrical permanent-magnet poles ofalternating polarities.
 4. A motor as claimed in claim 1, in which saidat least one pole group of the first pole line includes one symmetricalreluctance pole, one asymmetrical reluctance pole and one asymmetricalpermanent-magnet pole positioned between the reluctance poles, and inwhich the second pole line only comprises evenly spaced permanent-magnetpoles of alternating polarities.
 5. A motor as claimed in claim 1, inwhich said at least one pole group of the first pole line includes thetwo reluctance poles and one symmetrical permanent-magnet polepositioned between the reluctance poles.
 6. A motor as claimed in claim5, in which the second pole line only comprises a plurality ofsubstantially evenly spaced asymmetrical reluctance poles.
 7. A motor asclaimed in claim 5 in which the pole group of the first pole lineincludes the two reluctance poles and a permanent-magnet pole positionedbetween the reluctance poles, and in which the second pole line onlycomprises a plurality of permanent-magnet poles of alternatingpolarities.
 8. A motor as claimed in claim 1, in which the second poleline comprises a plurality of permanent-magnet poles of alternatingpolarities, like-polarity poles being substantially evenly spaced.
 9. Amotor as claimed in claim 8, in which the permanent-magnets of thesecond pole line are asymmetrical.
 10. A self-starting brushlesselectric motor, comprising a first motor part (11) having a plurality ofpole groups arranged in spaced-apart relation in a first pole line, asecond motor part (16) having a plurality of poles arranged inspaced-apart relation in a second pole line, bearing means supportingthe first motor part and the second motor part for relative movementwith the first pole line confronting the second pole line across an airgap, the first pole line and the second pole line comprising a polesystem comprising reluctance poles and permanent-magnet poles which arepolarized transversely to the air gap, and a winding system (15) on thefirst motor part comprising a winding coil arranged in association witheach pole group to produce a magnetic field linking the poles of thefirst pole line and the second pole line through the pole group uponenergization of the coil, at least one of the first pole line and thesecond pole line including a magnetic asymmetry providing a preferentialrelative direction of movement of the motor parts upon energization ofthe winding system, at least one of said pole groups comprising anasymmetrical permanent-magnet pole; in which said at least one polegroup of the first pole line includes at least one symmetricalreluctance pole; and in which said at least one pole group of the firstpole line includes one symmetrical reluctance pole, one asymmetricalreluctance pole and one asymmetrical permanent-magnet pole positionedbetween the reluctance poles, and in which the second pole linecomprises a plurality of substantially evenly spaced asymmetricalidentical poles.
 11. A motor according to claim 10, in which theasymmetrical permanent-magnet poles on the first motor part and on thesecond motor part comprise a main pole part and an auxiliary pole partwhich projects from the main pole part in the preferential relativedirection of the movement of the respective motor part.
 12. A motoraccording to claim 11, in which the auxiliary pole parts are of alength, as measured along pole lines, such that when any two poles onthe two motor parts are magnetically aligned with one another, theauxiliary pole part of at least one of the magnetically asymmetricalpoles on one of the motor parts extends to a vicinity of an adjacentpole on the other motor part.
 13. A motor according to claim 12, whereinthe auxiliary pole part of the at least one of the magneticallyasymmetrical poles on one of the motor parts slightly overlaps saidadjacent pole.
 14. A motor as claimed in claim 11, in which the polegroups of the first pole line are identical.
 15. A motor according toclaim 11, in which the motor is a rotary motor and in which the firstpole line comprises at least one pair of diametrically opposite polegroups.
 16. A self-starting brushless electric motor, comprising a firstmotor part (11) having a plurality of pole groups arranged inspaced-apart relation in a first pole line, a second motor part (16)having a plurality of poles arranged in spaced-apart relation in asecond pole line, bearing means supporting the first motor part and thesecond motor part for relative movement with the first pole lineconfronting the second pole line across an air gap, the first pole lineand the second pole line comprising a pole system comprising reluctancepoles and permanent-magnet poles which are polarized transversely to theair gap, a winding system (15) on the first motor part comprising awinding coil arranged in association with each pole group to produce amagnetic field linking the poles of the first pole line and the secondpole line through the pole group upon energization fo the coil, adirection of said magnetic field being opposite to a direction ofmagnetization of the permanent magnets, at least one of the first poleline and the second pole line including a magnetic asymmetry providing apreferential relative direction of movement of the motor parts uponenergization of the winding system, at least one of said pole groupscomprising a pair of reluctance poles and an asymmetricalpermanent-magnet pole positioned between and spaced from said reluctancepoles; and wherein the second pole, line includes at least oneasymmetrical reluctance pole said at least one pole group of the firstpole line includes two symmetrical reluctance poles, thepermanent-magnet pole being positioned between the reluctance poles, andin which the second pole line comprises a plurality of substantiallyevenly spaced asymmetrical identical poles; asymmetrical poles on thefirst motor part and on the second motor part comprise a main pole partand an auxiliary pole part which projects from the main pole part; theauxiliary pole parts are of a length, as measured along pole lines, suchthat when any two poles on the two motor parts are magnetically alignedwith one another, the auxiliary pole part of at least one of themagnetically asymmetrical poles on one of the motor parts extends atleast to the vicinity of an adjacent pole on the other motor part andpreferably slightly overlaps said adjacent pole; and the auxiliary polepart of at least one of the magnetically asymmetrical poles on one ofthe motor parts slightly overlaps said adjacent pole.
 17. A motor asclaimed in claim 16, in which said at least one pole group of the firstpole line includes at least one symmetrical reluctance pole.
 18. Aself-starting brushless electric motor, comprising a first motor part(11) having a plurality of pole groups arranged in spaced-apart relationin a first pole line, a second motor part (16) having a plurality ofpoles arranged in spaced-apart relation in a second pole line, bearingmeans supporting the first motor part and the second motor part forrelative movement with the first pole line confronting the second poleline across an air gap, the first pole line and the second pole linecomprising a pole system comprising reluctance poles andpermanent-magnet poles which are polarized transversely to the air gap,and a winding system (15) on the first motor part comprising a windingcoil arranged in association with each pole group to produce a magneticfield linking the poles of the first pole line and the second pole linethrough the pole group upon energization of the coil, at least one ofthe first pole line and the second pole line including a magneticasymmetry providing a preferential relative direction of movement of themotor parts upon energization of the winding system, at least one ofsaid pole groups comprising an asymmetrical permanent-magnet pole; inwhich said at least one pole group of the first pole line includes atleast one symmetrical reluctance pole; and in which said at least onepole group of the first pole line includes one symmetrical reluctancepole, one asymmetrical reluctance pole and one asymmetricalpermanent-magnet pole positioned between the reluctance poles, and inwhich the second pole line includes a plurality of substantially evenlyspaced symmetrical permanent-magnet poles.
 19. A self-starting brushlesselectric motor, comprising a first motor part (11) having a plurality ofpole groups arranged in spaced-apart relation in a first pole line, asecond motor part (16) having a plurality of poles arranged inspaced-apart relation in a second pole line. bearing means supportingthe first motor part and the second motor part for relative movementwith the first pole line confronting the second pole line across an airgap, the first pole line and the second pole line comprising a polesystem comprising reluctance poles and permanent-magnet poles which arepolarized transversely to the air gap, and a winding system (15) on thefirst motor part comprising a winding coil arranged in association witheach pole group to produce a magnetic field linking the poles of thefirst pole line and the second pole line through the pole group uponenergization of the coil, at least one of the first pole line and thesecond pole line including a magnetic asymmetry providing a preferentialrelative direction of movement of the motor parts upon energization ofthe winding system, at least one of said pole groups comprising anasymmetrical permanent-magnet pole; in which said at least one polegroup of the first pole line comprises a permanent-magnet pole which isplaced in an asymmetrical position, and the second pole line comprises aplurality of substantially evenly spaced asymmetrical reluctance poles;in which said at least one pole group comprises a pair of reluctancepoles, the permanent-magnet pole being placed between the reluctancepoles in a position which is asymmetrical with respect to them; and inwhich the permanent-magnet pole and the reluctance poles aresymmetrical, in which the permanent-magnet pole is displaceable alongthe pole line from said asymmetrical position to a correspondingasymmetrical position on the opposite side of a line of symmetry betweenthe reluctance poles and in which the reluctance poles of the secondpole line comprise a main pole part and an auxiliary pole part at eachend of the main pole part.